Non-aqueous electrolyte lithium secondary batteries are put to practical use as secondary batteries for portable electronic devices and are widely used. However, LiCoO2 which is a compound of Co, which is expensive and less in the deposit amount as resources, is used as the positive electrode active material of the batteries, and there has been demanded a positive electrode active material for non-aqueous electrolyte secondary batteries which comprises a compound containing mainly the abundant element as resources.
Thus, NaFeO2 which is a composite oxide of iron and sodium which are abundant elements as resources has been proposed as a positive electrode active material for non-aqueous electrolyte secondary batteries, and it is known that this compound is obtained by mixing Na2O2 and Fe3O4 and firing the mixture at 600-700° C. in the air (see, for example, Non-Patent Document 1).
Non-Patent Document 1: Materials Research Bulletin (U.S.A.), Pergamon Press, 1994, Vol. 29, No. 6, p. 659-666